Time series of optical satellite images acquired at high spatial resolution constitute an important source of information for crop monitoring, in particular for keeping track of crop harvest. However, the quantity of information extracted from this source is often restricted by acquisition gaps and uncertainty of radiometric values. This paper presents a novel approach that addresses this issue by combining time series of satellite images with other information from crop modeling and expert knowledge. An application for sugarcane harvest detection on Reunion Island using a SPOT5 time series is detailed. In a fuzzy framework, an expert system was designed and developed to combine multi-source information and to make decisions. This expert system was assessed for two sugarcane farms. Results obtained were in substantial agreement with ground truth data; the overall accuracy reached 96.07%.